The invention relates to a method of and an arrangement for measuring the properties, particularly the degree of compaction of a stiff mass to be compacted, such as fresh concrete to be cast and similar soil mass.
The use of concrete masses containing ever decreasing amounts of water has been on the increase in concrete industries. This is due to the fact that, generally speaking, the strength values provided by the mass are better, the lower the water content of the mass, provided that the mass can be compacted effectively. In practice, an amount of water of 20-30% of the amount of cement is sufficient for hydration, that is, the W/C ratio ranges from 0.20 to 0.30. Furthermore, cement is today ground very fine in order that the concrete strength to be obtained by it could be utilized better and more rapidly. Such masses, called soil moist masses, are difficult to mould and cast. Such masses are often called stiff masses.
Stiff masses are used, e.g., in modern slip casting processes, and in roll compaction. Even though there are various useful methods and apparatuses for measuring the properties of plastic masses, such as the degree of compaction, there are no suitable methods and apparatuses for accurately measuring the degree of compaction of stiff masses. The greater the degree of compaction of concrete in the finished cast product, the greater the strength obtained. Therefore, information on the compacting properties of a concrete mass to be cast is very important to obtain a good end result.
The compactability of concrete is substantially dependent on the "stiffness" of the mass, that is, in general, its water content. When the mass is too stiff, the cast has poor compacting properties and many air pores remain within the cast. When the mass is too plastic, it apparently contains excess water, which deteriorates the strength. The end result is inferior, and the shape and tolerances of the cast are difficult to control when casting without a fixed mould. The compactability of concrete is not only affected by the amounts of ingredients and the mix proportions, but also by the quality and size of the ingredients and other factors. For this reason reliable information on the compacting properties of the mass is of vital importance.
In the absence of suitable measuring methods and apparatuses, evaluation by feeling with the hands has been used previously in an attempt to measure the compacting properties of stiff concretes. Such evaluation, however, requires high professional skill and is nevertheless always unreliable and subjective and consequently, cast products are often rejected.
An example of more advanced measuring methods and apparatuses for stiff masses is disclosed in FI Patent Specification 71619. This method is an indirect method in which a sample is taken from the mass to observe the process. Increase in the density of the sample indicates an increased degree of compaction. The method is applicable only in cases where the properties are measured by sampling. Information provided by the method of FI Patent Specification 71619 is not sufficient for research work, for example, but information on other compacting properties is required. For example, variation in the quality of stone aggregate must be accurately determined and taken into account. The specific weight of the stone aggregate must be accurately determined as it affects the density measurements.
In addition to fresh concrete to be cast, there are other masses which are measured for their properties, such as soil and bitumen masses.
The object of the invention is to provide a method and an arrangement by means of which the drawbacks of the prior art can be eliminated, and which enables the degree of compaction of a stiff mass to be measured more accurately than was done previously. This is achieved by means of a method and arrangement according to the invention. The method is characterized in that gaseous pressure fluid is blown through the mass during the compaction of the mass, simultaneously observing increases in the pressure of the gaseous pressure fluid in the surface portions of the mass due to increasing flow resistance. The arrangement, in turn, is characterized in that it comprises; an opening formed in a wall adjoining the mass to be compacted, a source of pressure fluid connected to said opening so as to blow gaseous pressure fluid through the opening on to the surface of the mass and further therethrough, and a measuring device for measuring the pressure of the pressure fluid in the surface portions of the mass.
The invention is based on a certain property of a stiff mass to be compacted, that is, on the fact that stiff uncompacted concrete, for instance, contains about 20-50% by volume of air. To illustrate this, assume that concrete contains only large aggregate particles and moist cement paste. In a concrete mixer, the cement has spread everywhere on the surface of the aggregate particles. In uncompacted concrete, there are plenty of air cavities between the aggregate particles covered by cement paste. There are air cavities between the aggregate particles and the cement paste as well. These air cavities form a continuous three-axis network which extends throughout the concrete and in which air, for instance, can easily pass through the concrete. The invention is based on the fact that this network of cavities changes while the concrete is compacted, whereby the flow resistance of air passed through the concrete also changes in a determined manner.
An advantage of the invention is that it enables an efficient control of the compaction of mass, whereby the final product is of desired quality. This reduces the number of rejected products and the waste labour associated therewith. A further advantage of the invention is its wide range of applications: it can be applied both directly at the manufacturing stage, e.g., by utilizing a concrete mould, and with indirect methods known from the prior art. Still another advantage is that the whole mixing formula and the specific weights of the ingredients need not be taken into account in the measuring.
In the following, the invention will be described in greater detail with reference to the attached drawings illustrating an embodiment in which the invention is applied to a concrete mass, whereby: